OFDM transmissions are widely used. Examples of their use include Digital Video Broadcast (DVB), Digital Audio Broadcast (DAB), and wireless broadband transmission standards such as IEEE 802.11a, ETSI/BRAN/Hiperlan2 and ARIB/MMAC/HiSWAN.
An OFDM transmitter/receiver (‘transceiver’) includes an analogue signal processing part, the RF front-end, and a digital signal processing (DSP) part, also referred to as the base-band digital IC. In the reception direction, the function of the RF front-end is to convert the OFDM signal from the RF frequency (e.g. 5 GHz in IEEE802.11a) to base-band, and to generate the in-phase (I) and quadrature (Q) components of the base-band signal. The digitised I and Q signals are then processed by the DSP unit. There are two basic architectures to generate I and Q digital signals:                The first architecture, which is not utilised by the present invention, is known as digital I/Q generation. In digital I/Q generation, the RF signal is converted to a low intermediate frequency (IF) (for example 20 MHz in IEEE802.11a) and sampled at a relatively high frequency (e.g. greater than 40 MHz in IEEE802.11a) by a single Analogue to Digital Converter (ADC). The single digitised signal is then processed by the DSP unit, which digitally generates the I and Q signals and processes them. A drawback of this architecture is that it is power consuming and increases the complexity of the DSP. Also, many I/Q compensation techniques only deal with a mismatch that remains constant over the whole frequency band of the signal and only give good results for narrow band signals.        The second architecture is called analogue I/Q generation. An example of a transceiver in conformity with the IEEE802.11a standards with this architecture as disclosed in our co-pending European Patent Application N° EP 01401631.5 filed 20 Jun. 2001 is shown in FIG. 1 and the signals appearing in operation at various stages of the receiver are shown in FIG. 2.        
An analogue I/Q receiver may include a first down-conversion stage, which converts the RF signal to an intermediate frequency, as shown in FIGS. 1 and 2. However it is also possible for the receiver to convert the RF signal directly down to base-band.
The transceiver illustrated in FIGS. 1 and 2 comprises an analogue I/Q receiver section 1 and a transmitter section 2. In the analogue receiver section 1, the RF signal is filtered in a band-pass filter 3, amplified in a low-noise amplifier 4, converted down to intermediate frequency by mixing in a mixer 5 with an RF signal generated locally by a voltage controlled oscillator 6 and filtered in a band-pass filter 7. The IF signal is then mixed in two mixers 8 and 9 with two sine waves produced by a voltage controlled oscillator 10 and which have the same IF frequency with a phase difference of 90 degrees, so as to generate the base-band I and Q signals. The I and Q signals are filtered by respective low-pass filters 11 and 12. Then they are digitised using respective analogue-to-digital converters 13 and 14 and demodulated in an OFDM demodulator 15. The two ADCs are typically clocked at a frequency that is at least a factor of two lower than in digital I/Q receivers, which reduces the circuit area and power consumption and also simplifies the base-band digital IC compared to a receiver using digital I/Q generation.
Analogue I/Q generation has been found to be more difficult to implement than digital I/Q generation, however, because avoiding signal impairment (such as cross-talk between the sub-carriers especially, for example) has required high quality matching between the I and Q signal paths. The analogue treatment of the I/Q signals is sensitive to mismatch. Such mismatch arises from slight differences in the values and behaviour of active and passive elements found in the I and Q signal paths, even though great care is taken in the design and layout of these elements in a symmetrical way during the design of the system and/or circuit. Mismatches are even more pronounced when the effects of thermal drift are taken into account.
The present invention enables signal impairments to be reduced without requiring such high quality matching between the I and Q signal paths. The overall solution combines the advantages of a high quality signal and a low power consumption and circuit area.